1. Field of the Invention
The invention relates to the detection and documentation of latent fingerprints. More specifically, the invention relates to a method and apparatus for chemically imaging latent fingerprints and documenting the disclosed fingerprint images using conventional or electronic photographic techniques.
2. Background of the Invention
A fingerprint is perhaps the most powerful and valuable evidence capable of linking a suspect to the crime scene. Fingerprints which are found on surfaces can be generally classified into three types: visible, impression and latent fingerprints. Of these, visible fingerprints can be analyzed and documented by photographing them directly, while impression fingerprints can usually be photographed using special lighting techniques. Latent fingerprints, however, are difficult to detect and prior to documentation and analysis must first be made visible, that is, they must be xe2x80x9cimagedxe2x80x9d.
There are three general classes of techniques for making latent fingerprints visible: physical, chemical and instrumental techniques. This application is concerned primarily with chemical techniques for imaging and documenting fingerprints from the residues which form the latent fingerprint itself. These residues comprise a variety of substances from the body which are exuded through the skin of the fingertips or which are produced in other locations on the body and transferred to the fingers through contact with those areas.
The primary component of a fingerprint is ordinary perspiration. This mostly contains water which evaporates readily from a fingerprint and leaves a residue of various chemicals. The residue contains both inorganic and organic materials some of which can remain detectable on a surface long after the water component of the perspiration has evaporated. These chemicals include water soluble amino acids, peptides, salts, glucose, lactic acid, ammonia, riboflavin, and water insoluble oils and other sebaceous secretions (generally referred to as lipids). For the purposes of this application, the term xe2x80x9cfingerprintxe2x80x9d is used to describe both the chemical residue left when a person touches an object, or the image formed by the residue.
From the time the detection and use of fingerprints as a forensic tool began in the nineteenth century a variety of chemical methods have been developed which utilize the various substances contained in the residues of a latent fingerprint for creating an observable image. For example, silver nitrate was found to react with the salt in a latent print which, through exposure to an actinating light source, forms a visible fingerprint image. However, exposure to moisture can readily remove salts from a latent fingerprint making the print undetectable by this method.
In the ninhydrin technique the amino acids present in a latent fingerprint are reacted with triketohydrinden hydrate to create a visible purple-blue fingerprint image. The ninhydrin technique takes advantage of the reaction of the long-lasting and environmentally durable amino acids which may be present in a fingerprint. However, it is well known that not all fingerprints contain amino acids and consequently, this technique will not disclose all latent prints which may be present on a surface.
The lipids in a fingerprint are relatively durable and long-lasting when exposed to the environment. Lipids do not deteriorate as readily as salts through exposure to moisture and, unlike amino acids, lipids are always present in a fingerprint. Advantages to using lipids in the fingerprint as a basis for detection are well recognized and several methods have been developed as a result.
The method widely known as xe2x80x9cdusting for printsxe2x80x9d involves depositing a colored powder on a surface suspected of bearing latent prints. The powder adheres to lipid residue on a surface and the loose excess powder is delicately brushed off in a tedious and labor intensive process, thereby disclosing any latent fingerprints. The disclosed prints may then be physically xe2x80x9cliftedxe2x80x9d from the surface with adhesive tape and preserved on a piece of card stock.
In another method iodine crystals are warmed causing the sublimation of the crystals and the gas thus produced is blown or wafted over the surface being examined for latent prints. Iodine gas reacts with the lipids, causing the latent print to become visible. The fingerprint image produced by this method is evanescent and will fade, eventually vanishing over time. Photographic records of the freshly disclosed prints may be made, or chemical fixatives can be applied to halt the deterioration of the disclosed print. Iodine is a strong oxidizing agent and some forensic investigators may avoid its use as it causes rapid destructive rusting of metal.
Recent developmental work in the field of fingerprint detection has yielded new detection methods including various fluorogenic visualization and cyanoacrylate (C/A) fuming techniques.
In the fluorogenic visualization of amino acids, the latent image is treated with one or more chemical reagents which react with and covalently bond with compounds in the print to form a fluorescent chemical product. The image of the latent print is then viewed or photographed with the aid of an optical filter and under illumination of light of appropriate wavelength to cause excitation and fluorescence of the image. The enhanced detectability of latent images by application of fluorogenic rather than color-development techniques has stimulated research resulting in the development of a number of useful reagents for amino-acid fingerprint detection. Among these are ninhydryn, 5-methoxyninhydryn, 4-chloro-7-nitrobenzofurazan (NBD)-chloride, 1-8 diazafluorene-9-one (DFO), aminoninhydryns and 5-thioninhydryns.
Luminescent dyes are also used to enhance fingerprints developed for use in conjunction with C/A fuming techniques. Many dyes, such as Rhodamine 6G, Ardrox or Basic Yellow 40 are readily available and routinely used either to introduce contrast or to increase detection sensitivity in these techniques. Europium chelates have also been explored for this purpose.
Recently, a lipid-specific lanthanide-based method for latent fingerprint detection has been proposed. Although this method offers some promise for the detection of latent lipid images, its practical implementation is subject to a number of problems and drawbacks. One of the drawbacks is that the multi-step multi-reaction lanthanide chemistry used in the technique is complex. Lanthanide ions have very poor energy absorption and hence are not efficiently excited by energy sources. In order to enhance fluorescence the lanthanide ions are chelated with organic ligands which exhibit good energy absorption properties and are able to transfer their excitation energy to the lanthanide via resonant energy transfer. The application of this technique to the detection of latent lipid images is even further complicated in that it requires the lanthanide ion to be reacted with a conjugating ligand (non-luminescent) to provide some specificity toward lipids and further reaction with a sensitizing ligand to generate lanthanide luminescence through energy transfer.
Another drawback of the lanthanide-based method is that it is subject to high background fluorescence which reduces contrast and detection sensitivity of the process. The source of the high fluorescence background in the lanthanide-based method is two-fold. First, the method requires excitation via ultraviolet (UV) illumination and many surfaces on which fingerprints are located, for example, on biological surfaces (leaves, wood, etc.), plastic, paper, glass, and a host of other materials exhibit some degree of background fluorescence when excited by energy in the UV and short-wavelength visible spectrum range. Second, the sensitizing ligand required to generate lanthanide luminescence itself causes non-specific background fluorescence. The long luminescence lifetime of lanthanide chelates (hundreds of microseconds to milliseconds in duration) permits, in principle, the reduction of background luminescence through time-resolved detection techniques. The time-resolved techniques require pulsed illumination (e.g., a pulsed argon ion laser) with a delay-gated detection device. It will be well understood that these detection techniques introduce additional complexity and expense to forensic work.
The lanthanide-based method is even further restricted in its use according to the age of the print being detected. It appears that the lanthanide method is only suitable for use on prints which are quite fresh and works poorly, if at all, on older fingerprints.
The limitations and shortcomings of the various known techniques coupled with the great potential in using the lipid residues present in a latent fingerprint for forensic analysis have created a pressing need for development of better detection methods. Ideally, a detection method should be capable of generating a lipid-specific fluorescent image with low background fluorescence and a high contrast image. The technique should provide a high detection sensitivity, use a simple one-step chemical process, and create images amenable to recording with inexpensive, largely conventional cameras and optical equipment and simple, low-power illumination sources. Furthermore, it is advantageous for the method to generate and capture images in the presence of full ambient illumination on site whether the light is natural sunlight, incandescent, fluorescent, or from another light source.
An object of the present invention is to provide a method for the detection of latent lipid images such as those formed by fingerprints.
Another object of the invention is to provide a method for the detection of latent lipid images using a dye which fluoresces on exposure to an appropriate light source when the dye is in contact with a lipid, but in the absence of contact with a lipid the dye fluoresces little or not at all.
Yet another object of the invention is to provide a method for the detection of latent lipid images using a dye which does not require chemical reaction with, or covalent bonding with the lipid in order for detection of the lipid to occur.
A further object of the invention is to provide an apparatus for detection of latent lipid images.
Still another object of the invention is to provide a method of taking and recording distinctive patterns and markings of the skin, including fingerprints, footprints and handprints for identification purposes.
Other objects of the invention will be in part obvious and in part pointed out more in detail hereinafter.
These and other objects of the invention are achieved through the method of applying a dye to a surface having a latent pattern formed of lipid material. The dye is characterized as having an affinity for lipids and for fluorescing when in contact with a lipid and exposed to an appropriate energy source, thereby forming a fluorescing image of the latent lipid pattern. Viewing of the fluorescing image may be accomplished using an optical filter appropriate for detecting the wavelength of the fluoresced light. The revealed image may also be observed, recorded and preserved by any conventional film or digital camera or other imaging means with the use of film, filters or conventional optical equipment appropriate for recording the particular wavelength light which is fluoresced by the image.
The dyes having utility in the invention are dyes which exhibit sensitivity to the environment in which they are present, that is, the dyes are capable of fluorescence when in contact with lipids, but possess little or no ability to fluoresce when in a non-lipid environment such as water or polar solvents. The dyes having particular utility in the invention are dyes in the styryl family of dyes.
The invention accordingly comprises the several steps and the relation of one or more steps to the others, and the apparatus possessing the features, properties, and relation of elements as set forth in the following disclosure.